Animal only
This mouse study examined how acute restraint stress (30 minutes) interferes with hunger-driven food-seeking behavior and what interventions can reverse this effect. Using a conflict-based open-field feeding paradigm, researchers found that restraint stress abolished the increases in food seeking and intake normally observed in fasted mice. Two pharmacological interventions — intraperitoneal diazepam (an anxiolytic) and MK-677 (a ghrelin receptor agonist) — both reversed this stress-induced suppression of feeding behavior, though through apparently distinct neural mechanisms. To map neural correlates, the study measured c-Fos (a marker of neuronal activation) and phosphorylated pyruvate dehydrogenase (pPDH, a marker of neuronal inhibition) in the paraventricular hypothalamic nucleus (PVN). Diazepam reduced restraint-induced c-Fos expression in the PVN, suggesting anxiolysis works via suppressing stress-driven PVN activation, while MK-677 increased pPDH, indicating a separate feeding-drive mechanism. Notably, refeeding after fasting produced a pPDH-dominant PVN pattern and also reduced anxiety-related behaviors, suggesting physiological restoration of energy balance may itself confer stress resilience. Key limitations include exclusive use of a mouse model, the use of a single stress paradigm, and the lack of direct causal circuit manipulations. Findings are not directly applicable to humans without further study.
Neuroscience research · Nov 2025DOI ↗ Animal only
This mouse study investigated whether intranasal administration of ghrelin, GHRP-6, or MK-0677 could activate the brain's ghrelin signaling system. Researchers first screened compounds and doses by measuring food intake after intranasal application. Of the three compounds tested, only GHRP-6 at a specific dose increased food intake without adverse effects and was selected for detailed analysis. Investigators then examined meal patterns, neuronal activation in the arcuate nucleus of the hypothalamus using Fos mapping, and neurochemical identity of activated neurons using RNAscope in situ hybridization. They also measured serum growth hormone (GH) levels. The study found that intranasal GHRP-6 increased food intake by raising both meal frequency and meal size. Fos expression in the arcuate nucleus was significantly elevated compared to saline controls, and activated neurons showed co-expression with GHSR, AgRP, and GHRH mRNA markers, implicating circuits involved in feeding and GH regulation. Serum GH levels were also elevated following intranasal GHRP-6. Limitations include exclusive use of a mouse model, a single species and sex are not specified, and the absence of human or pharmacokinetic data, meaning translation to clinical settings remains uncertain.
Endocrinology · Feb 2025DOI ↗ Animal only
This study investigated whether MK-0677 (ibutamoren), an orally active, non-peptide growth hormone secretagogue that activates the ghrelin receptor, could be detected in equine hair following oral administration to a single Thoroughbred racehorse. Researchers extracted MK-0677 and its O-dealkylated metabolite from mane and tail hair samples using an established method for prohibited substances, then analyzed them by liquid chromatography tandem mass spectrometry (LC-MS/MS). The study found that MK-0677 was detectable in all collected hair samples, with a detection window of up to 209 days in mane hair and 358 days in tail hair. A follow-up wash procedure confirmed true internal incorporation of the compound rather than mere surface contamination. Wash criteria analyses suggested that hair samples collected at later time points (≥52 days post-administration) reflected internal incorporation via the bloodstream, while the earliest sample (2 days) showed a combination of internal incorporation and external deposition via sweat and sebum. A key limitation is that the study involved only one horse, restricting generalizability. The findings are relevant to anti-doping efforts in equine sports, highlighting the long detection window MK-0677 may afford in hair matrices.
Drug testing and analysis · Dec 2022DOI ↗ Animal only
This study investigated the metabolic profile of ibutamoren (MK-0677) — an orally active, non-peptide growth hormone secretagogue — in thoroughbred horses, with the goal of supporting anti-doping detection efforts. Researchers administered ibutamoren orally to horses and collected biological samples, which were analyzed using liquid chromatography coupled with high-resolution mass spectrometry (LC-HRMS) to identify and characterize the parent compound and its metabolites. The study identified 22 metabolites in total: 17 Phase I metabolites (including mono- and di-hydroxylated forms, dissociated side chain products such as a benzyl group and 2-amino-2-methylpropanamide, and hydrogenated metabolites) and 5 Phase II metabolites (glucuronic acid conjugates; no sulfate conjugates were detected). The study reported that major metabolites were detectable up to 96 hours post-administration, while the parent compound ibutamoren itself persisted for up to 72 hours. The authors conclude that these findings provide a useful metabolic reference framework to help detect illicit use of ibutamoren in competitive equine sports. Key limitations include the use of an animal model (horses only), meaning findings may not directly translate to human metabolism or human anti-doping contexts.
Rapid communications in mass spectrometry : RCM · Sep 2022DOI ↗ Animal only
This study investigated the metabolism of MK-0677 (ibutamoren mesylate), an orally active non-peptide growth hormone secretagogue, in horses to support equine doping control efforts. Researchers conducted both in vitro incubations and in vivo oral administration to two Thoroughbred racehorses, then used liquid chromatography high-resolution mass spectrometry (LC-HRMS) and LC-tandem mass spectrometry to identify and profile metabolites in urine and plasma. Fourteen phase I metabolites were identified in vitro; 13 were subsequently detected in post-administration urine and nine in plasma, along with the parent compound in both matrices. The study found that an O-dealkylated metabolite of MK-0677 showed the longest detection window in both urine and plasma, making it the recommended analytical target for doping control laboratories. Both MK-0677 and this key metabolite were found to be excreted predominantly in unconjugated form. Limitations include the very small sample size (n=2 horses) and the fact that findings are restricted to equine species, meaning results cannot be directly extrapolated to human metabolism or doping detection in human sport.
Drug testing and analysis · Mar 2022DOI ↗